Jet Physics with ALICE Oliver Busch for the ALICE collaboration Oliver Busch Tsukuba 2014 /03/13 1
Outline introduction results from pp jets in heavy-ion collisions results from Pb-Pb collisions jets in p-pb collisions outlook: LHC run 2 Oliver Busch Tsukuba 2014 /03/13 2
Introduction Oliver Busch Tsukuba 2014 /03/13 3
Jets: seeing quarks and gluons jet: collimated spray of hadrons quasi-free parton scattering at high Q 2 : the best available experimental equivalent to quarks and gluons Oliver Busch Tsukuba 2014 /03/13 4
Jet fragmentation initial hard scattering: high-p T partons cascade of gluons: parton shower at soft scale (O(Λ QCD )): hadronization Oliver Busch Tsukuba 2014 /03/13 5
LHC aerial view Oliver Busch Tsukuba 2014 /03/13 6
Jets at ALICE (LHC run 1) charged particle tracking: - Inner Tracking System (ITS) - Time Projection Chamber - full azimuth, η < 0.9 p T > 150 MeV/c EMCal : - neutral particles - Δφ = 107, η <0.7 cluster E T > 300 MeV jet trigger with EMCal and TRD `charged (tracking) jets and `full jets full jets from charged particle tracking and EM energy: conceptually different and complementary to traditional approach 7
Results from pp collisions Oliver Busch Tsukuba 2014 /03/13 8
Full jets in pp at = 2.76 TeV good agreement to NLO calculations for R = 0.2 and R = 0.4 reference for Pb-Pb at same energy R = 0.2 R = 0.4 Phys. Lett. B 722 (2013) 262 Oliver Busch Tsukuba 2014 /03/13 9
pp charged jet cross-sections measured in minimum bias collisions at = 7 TeV good agreement with ATLAS charged jet measurements (despite slightly different acceptance and track p T range) nucl-ex/1411.4969 R = 0.4 R = 0.6 Oliver Busch Tsukuba 2014 /03/13 10
pp jet fragmentation at = 7 TeV distributions of charged particles in charged jets scaling for z > 0.2 bulk production at low z: ~ 5-10 charged particles per jet nucl-ex/1411.4969
PID in jets : `TPC coherent fit particle identification via specific ionization in TPC ('de/dx'): TPC coherent fit: use energy loss model parameterization as input, adjust model parameters and particle fractions on the fly during fit regularization requiring continuity of particle fractions complementary and consistent: multi-template fit Oliver Busch Tsukuba 2014 /03/13 12
Particle identified fragmentation identified charged hadrons in charged jets at = 7 TeV π, K, p, 5 < p T ch jet < 20 GeV/c scaling for z ch > 0.2 for higher jet p T bins Oliver Busch Tsukuba 2014 /03/13 13
Particle ratios in jets strangeness content strongly enhanced for z ch 1 leading baryons suppressed Oliver Busch Tsukuba 2014 /03/13 14
Event generator comparison comparison to PYTHIA (p T ordered parton shower, Lund string fragmentation) data reasonably well described best reproduced by Perugia tune without color reconnections
Jets and Quark-Gluon Plasma Oliver Busch Tsukuba 2014 /03/13 16
QCD phase transition in heavy-ion collisions at ultra-relativistic energies, a quasi macroscopic fireball of hot, strongly interacting matter in local thermal equilibrium is created lattice QCD predicts phase transition to deconfined, chirally symmetric matter HotQCD, PRD 90, 094503 energy density from the lattice: rapid increase around T C, indicating increase of degrees of freedom (pion gas -> quarks and gluons) T C = 154+/- 9 MeV E C = 340 +/- 45 MeV/fm 3
Partons in heavy-ion collisions hard partons are produced early and traverse the hot and dense QGP enhanced energy loss: `jet quenching vacuum expectation calculable by pqcd : calibrated probe of QGP jets sensitive to properties of the medium (energy density,, mfp, coupling... )... but also jet/medium interaction not trivial (strong / weak coupling, parton mass / type, fireball dynamics... )
Hadrons in heavy-ion collisions hard partons `proxy for jet jet quenching for charged hadrons, Pb-Pb collisions at s NN = 2.76 TeV PLB 720 (2013) 250 hadron observables biased towards leading fragment study the effect for fully reconstructed jets Oliver Busch Tsukuba 2014 /03/13 19
Underlying event in heavy-ion collisions jet reconstruction in heavy-ion collisions : difficult due to the high underlying event background not related to hard scattering correct spectra for background fluctuations and detector effects via unfolding not possible down to 0 p T central jet area ~ 0.5 (R = 0.4) peripheral
Jet nuclear modification factor strong suppression observed, similar to hadron R AA -> parton energy not recovered inside jet cone increase of suppression with centrality nucl-ex/1502.01689 JEWEL: PLB 735 (2014) YaJEM:PRC 88 (2013) 014905 mild p T dependence JEWEL and YaJEM jet quenching models reproduce suppression
Jet structure jet structure ratio R=0.2 / R=0.3 for charged jets sensitive to potential broadening of jet shape consistent with PYTHIA pp: no modification observed within small radii (jet core) JHEP 03 (2014) 013
Results from p-pb collisions Oliver Busch Tsukuba 2014 /03/13 23
Charged jets in p-pb collisions p + Pb collisions at s NN = 5.02 TeV check potential initial state effects (nuclear modification of PDF) and effects of cold nuclear matter nucl-ex/1503.00681 measured spectrum consistent with POWHEG NLO calculations ( = NLO + PYTHIA fragmentation) Oliver Busch Tsukuba 2014 /03/13 24
Charged jet R p-pb R p-pb using scaled charged jet spectrum measured at 7 TeV as reference no jet suppression observed in p-pb -> suppression in Pb-Pb is final state effect nucl-ex/1503.00681 Oliver Busch Tsukuba 2014 /03/13 25
Jet structure in p-pb `jet structure ratio R=0.2 / R=0.4 for different centrality classes no modification, even for most central p-pb Oliver Busch Tsukuba 2014 /03/13 26
Perspectives for LHC run 2 Oliver Busch Tsukuba 2014 /03/13 27
LHC run 2 LHC run 2: 2015-2017 increased CMS energy for Pb-Pb collisions from 2.76 5.1 TeV quenching strength ~ s ~ ε 3/4 expect (modest) increase in ε, T measure energy density dependence of jet quenching ALICE, PRL 105, 252301 note: also a dependence on parton input spectrum (increased R AA??? )
ALICE in run 2: DCal run 2: DCal upgrade - significantly extended jet acceptance - back-to-back in azimuth (di-jet topology) Oliver Busch Tsukuba 2014 /03/13 29
Summary jet cross sections and properties in pp identified jet fragmentation in pp strong jet suppression observed in Pb-Pb collisions p-pb results: initial-state and cold-nuclear-matter effects negligible looking forward to LHC run 2! Oliver Busch Tsukuba 2014 /03/13 30
Jet reconstruction Establish correspondence between detector measurements / final state particles / partons two types of jet finder: - iterative cone - sequential recombination (e.g. anti-k T ) resolution parameter R hep-ph/0802.1189
Jet finder comparison k T : sequential recombination SISCone: cone algorithm nucl-ex/1411.4969 32
Multi Template Fit TPC multi-template fit - best possible description of de/dx from external reference - parametrize dependences on η, TPC nclusters - templates in transverse momentum (z, xi) slices de/dx in one z slice (0.6 < z < 0.65), 10-15 GeV/c fitted with 4 templates 33
Method comparison uncorrected hadron fractions from Multi-Template Fit and TPC Coherent Fit 2 complementary methods obtain consistent results
pp jet structure at = 7 TeV transverse structure: p T sum in radial slices increasing collimation for higher p T jets fragmentation and jet structure reasonably described by event generators nucl-ex/1411.4969 35
QCD matter at LHC direct photons: thermal radiation from the early stage of the fireball indicates initial temperature way above T C 36
Reaction plane dependence different medium thickness in- and out-of plane sensitive to path length dependence of jet quenching pqcd radiative E-loss : ~L 2 collisional E-loss : ~L strong coupling (ADS/CFT) : ~L 3 37
Local background subtraction prerequisite: take event plane dependence of background density into account event-by-event local ρ fit 38
Jet v2 : results charged jets, R = 0.2 quantify azimuthal asymmetry via 2 nd Fourier harmonic v2 strongly hints to non-zero jet v2 39
Recoil jets uncorrected jet semi inclusive recoil jet spectra : - reduce fake jet background through dijet topology - residual combinatorics subtracted in Δ recoil observable trigger hadron surface biased maximize path length for jet
Δ recoil fully corrected Δ recoil : R=0.2 and R=0.4 measurement down to constituent p T of 150 MeV/c, for large radii (up to R = 0.5) and low jet p T, without biasing leading constituent 41
ΔI AA ΔI AA : compare to PYTHIA pp reference suppression observed 42
Jet structure ratio Δ recoil Ratio R=0.2 / R=0.5 : sensitive to potential broadening of jet structure consistent with PYTHIA within large experimental uncertainties 43